1. Field of the Invention
The present invention relates to X-ray CT (computed tomography) apparatus, a method of aligning the phantom, and a phantom retaining tool, and more specifically relates to technology for aligning the phantom for X-ray CT apparatus.
2. Description of the Related Art
X-ray CT apparatus generates X-rays using an X-ray generator, and the X-rays that transmitted through an object on a bed are detected by an X-ray detector to form X-ray projection data, and it generates 2-dimensional tomographic image data showing the internal morphology of the object by applying a reconstruction processing to the X-ray projection data. Furthermore, with X-ray CT apparatus capable of a so-called helical scan, multiple tomographic image data can be reconstructed with a single scan by conducting a scan by rotation of an X-ray generator and an X-ray detector inside a gantry as the bed is being moved in the direction of the body axis (slice direction) of an object.
In order to obtain high-precision tomographic image data, it is necessary to maintain good performance of the apparatus by calibrating the X-ray CT apparatus. Therefore, it is necessary to adjust the apparatus by evaluating performance when the apparatus is shipped and installed, and even regularly after being put into operation. In addition, noise, contrast scale, spatial resolution, thickness of slice, high-contrast resolution, and low-contrast resolution may be listed as evaluation items.
Evaluation of X-ray CT apparatus is performed by using a pseudo body that simulates the human body, called a phantom, instead of an actual human body. FIG. 1A shows the appearance of the phantom 100. The phantom 100 comprises a cylindrical case 101. The cylinder axis of the case 101 is indicated by a symbol J. This cylinder axis J is the axis of rotational symmetry of the cylindrical body (case 101).
FIG. 1B shows a cross-section 110 of the phantom 100 in an arbitrary slice position (excluding proximity of both ends). The case 101 of the phantom 100 is formed in a hollow center. The interior of the hollow center is filled with a filling 102 such as water.
In order to ensure accuracy of a performance evaluation using a cylindrical phantom 100, the phantom 100 must be placed in a suitable setting position. To that end, the cylinder axis J of the phantom 100 must match the slice direction. Moreover, it is necessary to place a cylinder axis J on the center of the scan (center of rotation of the X-ray generator and X-ray detector), as well as to place the phantom 100 in the center of the examination region.
The process of installing the phantom 100 is as follows. First, a phantom 100 is placed on a bed and tomographic image data is reconstructed. The operator adjusts the window width and/or window level so that an outline of the phantom 100, or in other words, the case 101 is displayed clearly, as the operator observes the image of the displayed tomographic image data (tomographic image). Then, a cross-scale (intersection across the center of the display screen indicates the center of the scan) is superimposed on the tomographic image to be displayed. The operator specifies, by visual measurement, to what degree the center position of the case 101—which is indicated in the tomographic image—is displaced and in which direction from the intersection of the cross-scale. Then, as the setting position of the phantom 100 is moved again, the tomographic image is observed to check the position after the move. This type of serial operation is repeated in order to set the phantom 100 in the targeted position.
A method for detecting the position of a phantom to confirm that the phantom is suitably set is disclosed in Japanese Unexamined Patent Application Publication 2001-314397. The detection method described in this literature comprises: scanning a phantom positioned on the center axis of rotation (center of the scan) of an X-ray imaging system along the body axis with a horizontal view; comparing each of the obtained projection data and the predetermined threshold value to extract the outline shape of the phantom in a 2-dimensional projection image region; and detecting the vertical gradient of the phantom based on the extracted outline shape. In other words, in this conventional method, the posture to install is detected by detecting the gradient of a lateral image (scanogram) of the phantom.
With the conventional method of setting a phantom, because the distance to move the phantom is visually specified by the operator, it is difficult to set a phantom in the targeted position with one attempt of the above-described serial operation, and this serial operation is normally repeated many times. Therefore, the operation of setting a phantom becomes a prolonged process, which is a heavy burden on the operator.
Furthermore, to set a phantom in the targeted position, the position of the phantom must be moved 2-dimensionally. Therefore, it can be relatively quick for a skilled operator to grasp a direction and distance of movement, but for operators who are not as skilled, such an operation is difficult. In addition, this kind of operation depends upon the sense of the operator, and some operators do not improve their skill with experience.
On the other hand, in the invention disclosed in Japanese Unexamined Patent Application Publication 2001-314397, the posture is detected with the assumption that the cylinder axis of the phantom is placed on the center of the scan. Thus, when such a method is used, the operation of matching the cylinder axis of the phantom with the center of the scan must be manually performed beforehand, so time and effort are required. Moreover, these hitherto known inventions had the problem of being unable to confirm whether the phantom is positioned at the center of the scan.
Furthermore, the invention disclosed in Japanese Unexamined Patent Application Publication 2001-314397 uses only the result of lateral scanning of the phantom, so it can detect only the vertical gradient. In the actual positioning operation, the phantom could lean horizontally, but it could not be handled by the existing inventions.